It is conventional to use servo motors for executing positioning functions, when precision is required. Such motors have substantial torque even at zero speed, so that small errors in position can be corrected by causing the servo motor to exert a sufficient torque in the required direction to reduce the error. Such motors are relatively expensive, and the control units for controlling and driving them are also relatively expensive.
Alternating current motors are relatively inexpensive, compared to servo motors, but they typically have very low torque at zero speed, so they have not been employed for precise positioning functions.
It is therefore desirable to provide a control system incorporating an inexpensive a.c. motor for executing precise position functions, instead of a more expensive servo motor and control system therefor.